The present invention is directed to copper-tin alloys which are especially suitable for use in the manufacture of structural parts which are joined together through the use of heat.
Copper-tin alloys have, due to their high mechanical strength and great resistance to sliding stress or wear and corrosion, been utilized for many different mechanical structural parts and preformed articles that are to be manufactured into semifinished products by mechanical working. Copper-tin alloys also have been used as casting materials and as wrought materials. Phosphor bronzes are also widely used due to their ready availability and low cost and have the physical properties of a high mechanical strength and ductility. Additionally, they offer a high corrosion resistance in many different environments.
Workable copper-tin materials are particularly attractive for use in the manufacture of structural parts having small dimensions and complicated geometries. For example, in DIN 17662, a wide variety of uses for 4 to 8% bronze is disclosed, which in addition to up to 8.5% tin, also contains phosphorus in an amount of from 0.01 to 0.35%, iron in an amount of up to 0.1%, nickel in an amount of up to 0.3%, zinc in an amount of up to 0.3% and lead in an amount of up to 0.05%. Improvements in these materials have been desired with respect to electrical conductivity and suitability for electromechanical structural parts.
WO 9/20176 and WO 98/48068 are concerned with the improvement of electrical conductivity and relaxation resistance of traditional copper-tin materials. However, these improvements have little bearing on the suitability of the use of copper-tin alloys in machine- and apparatus-building industries, and precision-mechanics and jewelry industries. In these particular industries, classic phosphorus-bronzes are still exclusively used due to the fact that these materials can be used in a wide variety of manners due to the characteristics which are obtained through cold-working. However, these classic phosphorus-bronzes also have their deficiencies.
Due to the manufacture of functional parts, it is often necessary to join different structural elements. Welding and hard soldering methods are typically utilized to join these structural elements or parts. However, due to the heat entering into the structural parts to be joined, losses in strength result in the parts of the metal exposed to the heat due to conservation and recrystalization. This is especially true when using fusion-welding and hard-soldering methods. In order to keep the loss in strength as small as possible, hard-soldering instead of welding is used as often as possible. With solders having operating temperatures typically starting at about 450xc2x0 C., the joining of the structural elements can be performed but this requires a compromise between high strength and good loading capacity.
Since solder serves as a filler metal, the strength of the solder plays a role in the mechanical stability of the joined structure. As such, high strength solders are desirable. However, high strength solders, as a rule, have higher melting temperatures. This results in an increase in the heat applied to the joined parts and an attendant loss in strength in the areas adjacent the soldered junction. As such, there is a need for materials which resist softening during soldering operations.
In the eyeglass industry, nickel-free materials have been developed as materials having a higher resistance to softening. Many different copper-aluminum and copper-titanium alloys have been formulated. These alloys offer better spring characteristics and resistance to softening than phosphor bronze alloys typically utilized for the bows of glasses. However, during the use of these nickel-free alloys, it has been found that hard soldering under a protective gas creates problems in that these materials also react with an oxygen-deficient atmosphere and thereby significantly hinder the wetability of the surfaces of the structural part with the solder. Good processability during hard soldering is only possible through the use of aggressive flux agents. However, these aggressive flux agents have problems with respect to work safety and environmental contamination and also may cause a color change and leave residues on the joined structural parts. This requires that cleaning be performed in utilities where appearance is important. Moreover, independent of the flux agent, copper-tin alloys also have a tendency to change color during heating which also requires a cleaning of the joined structural parts. These cleaning operations are expensive and highly undesirable.
As discussed above, copper-tin wrought alloys containing about 8 wt. % tin are easily formed and especially suitable for the manufacture of complex functional parts. These alloys are used as friction bearings and gearings, springs and for parts which are stressed by ocean water, such as chains, armatures, etc. When utilized as structural parts which are subjected to very high mechanical stresses, such as gears, copper-tin cast alloys with tin contents above 10% by weight are preferred. These cast bronzes are increased in mechanical strength through the increased tin content. However, the increased tin-content results in brittle phases being formed in the primary structure during the solidification in common casting. These phases are not removed, even through a thermal after-treatment, without pores or imperfections remaining in the materials, which also in turn influence reforming.
Therefore, there exists a need for material which combines the chemical and mechanical characteristics of casting bronzes with the processing characteristics of wrought materials having a cold-working ability and guarantee of a high mechanical strength and hardness. In order to meet this need, an alloy has been proposed which is a copper-tin alloy containing tin in an amount of from 12 to 20 wt. % to enhance the strength of the material with the remainder being copper. This alloy can be formed by spray compacting or band casting and then quickly cooled from the molten state to suppress precipitation. This results in the primary structure of the alloy at room temperature being free of microscopic precipitation and the preforms manufactured from these alloys can be hot or cold formed in an excellent manner.
Even though the copper-tin alloy disclosed above has advantageous properties, deficiencies still remain with the alloy. As in a case of conventional low tin content copper-tin wrought alloys, there is a need to deoxidize the melt. Elements having an affinity for oxygen, such as phosphorus, are added to the melt as with conventional alloys. Due to the high affinity for oxygen, these added elements have a tendency to burn off and form slag during melting and casting which requires a complicated post treatment in order to maintain the desired concentrations. Additionally, the oxides of the deoxidation media influences the melt in general and the melt viscosity in particular and thus can have an influence on the forming process, such as spray compacting. Oxides from the oxygen affinity added mixtures can also be created during the hot-forming of the copper-tin alloys and these oxides worsen the surface quality of the formed goods and result in contamination of the tool and shortens the life of the tool. The presence of these oxides in the formed material are also undesirable during cutting or chipping since, due to their hardness, they contribute to an increased wear of the tool.
As such, there is a need for materials, which are at least equal to the high tin content copper-tin alloys in mechanical strength, formability and corrosion resistance and yet can be handled in a simplified manner during manufacture and processing. There also is a need for materials, which on the one hand meet the requirements regarding strength and softening characteristics for alloys used in the manufacture of components which are joined by a heat treatment and yet offer the advantages of hard-solderable tin-bronzes.
It is an object of the present invention to provide a copper-tin alloy having a mechanical strength, formability and corrosion resistance at least equal to that of high tin content copper-tin alloys and yet can be handled in a simplified manner during manufacture and processing.
It is a second object of the present invention to provide a copper-tin alloy which has the strength and softening characteristics necessary for it to be used in the manufacture of component parts which are joined together by heat and yet offer the advantages of hard-solderable tin-bronzes.
These and other objects of the present invention are accomplished by providing a copper alloy comprising from 4 to 20 wt. % tin, 0.1 to 5 wt. % in total of at least one of iron and cobalt, other optional metals, and the balance being copper. The present invention also is directed to a method of manufacturing structural parts which are joined together through the use of heat and in which at least one of the structural parts is made of a copper alloy comprising from 4 to 20 wt. % tin, 0.1 to 5 wt. % in total of at least one of iron and cobalt, other optional metals, and the balance being copper.
Another aspect of the present invention provides a copper-tin alloy having a tin content of from 12-20% and an iron content of from 0.2 to 5% which can be used in the manufacture of mechanical structural parts of the machine-building or automotive industry.
Still another aspect of the present invention is directed to a copper-tin alloy containing 4 to 12 wt.% tin, 0.1 to 4 wt. % iron and 0.01 to 0.6 wt. % titanium.
These and other aspects of the present invention will be explained in more detail in the following discussion.
The copper-tin alloy, in one embodiment of the present invention, contains from 4 to 12 wt. % tin, 0.1 to 4 wt. % iron and 0.01 to 0.6 wt. % titanium, with the balance being copper. This alloy composition has a particularly high strength and resistance to softening. A particularly advantageous alloy composition results from alloying the titanium in a mass ratio of iron to titanium xe2x89xa72.5. Since titanium is an alloy element which easily reacts with oxygen in the presence of heat to form oxides which result in coatings which drastically reduce the wetability with molten solders, it is unexpectedly surprising that the addition of titanium would be favorable. That is, it has been shown that a copper-tin alloy containing 4 to 12 wt. % tin and 0.05 wt. % titanium has dramatically lower solder wetability. The soldering process can only be successfully performed with the aid of fluxing agents. However, when titanium is added in the ratio of the invention with iron to the alloy of the present invention, the soldering ability is not affected and the softening characteristics of the alloy is unexpectedly improved. The addition of titanium results in a significant delay in the onset of softening which results in decreased reproducibility in industrial hard-soldering operations and optimization of the mechanical strength of a soldered joint.
The titanium can be partially or totally replaced in the alloy by zirconium and hafnium and not adversely affect the alloys"" properties. Additionally, to reduce the cost of the alloy, copper can be partially replaced with at least one of manganese and zirconium. However, no more than 10% by weight copper should be replaced by these metals since a greater replacement amount makes the casting ability more difficult and clearly lessens the good corrosion characteristics of the alloy of the present invention. Phosphorus should not be added to the copper-tin alloys of the present invention when titanium is present. The addition of phosphorus when titanium is present in the alloy composition results in the production of needle-shaped titanium phosphides in the molten alloy which makes the semifinished product manufacture process very difficult and is detrimental to the overall mechanical characteristics of the alloy.
In another embodiment of the present invention, the copper-tin alloy contains from 4 to 12 wt. % tin and 0.1 to 4 wt. % iron. Phosphorus also can be present in the inventive alloy in an amount of up to 0.5 wt.%. Phosphorus causes a moderate increase in the mechanical strength of the alloy after cold-working. Whenever it is considered that deoxidation is necessary, a phosphorus content of at least 0.01 wt. % should be used. However, phosphorus in an amount above 0.5 wt. % should be avoided since scales produced during soldering operations in an oxygen-containing atmosphere have a tendency to break off. Moreover, high phosphorus concentrations reduce the ductility of the alloys. Additionally, in the presence of iron, high phosphorus contents lead to the formation of rough iron phosphide particles which may interfere with the building of the structure. Therefore, phosphorus should be present in a mass ratio of iron to phosphorus of 2/1 in order to insure a favorable structure of the alloy through freely precipitating iron. In order to reduce the cost of the alloy, the copper can be partially replaced by at least one of manganese and zirconium. However, no more than 10% by weight of copper should be replaced by these metals in order to avoid a deterioration in the casting ability and corrosion resistance characteristics of the alloy.
A semifinished product manufactured out of the alloy of the second embodiment of the present invention can be easily handled without any problems during the manufacture thereof through conventional forming and reforming processes. Additionally, the alloy has excellent hard-soldering characteristics with many different solders and no oxides are produced on the surface of the alloy which would cause a poor wetting ability or a poor solder flow. As such, this alloy is particularly suitable for use in the manufacture of structural parts which are joined by heat such as jewelry, clothing accessories and components of eyeglass frames.
In another aspect of the present invention, the copper-tin alloy contains tin in an amount of from 12 to 20 wt. % and iron in an amount of from 0.1 to 4 wt.%. This alloy is also particularly suitable for use in the manufacture of structural parts which are to be joined through the use of heat. The high tin content and presence of iron gives the inventive alloys a particularly high strength and resistance to softening and a deoxidation aid, such as phosphorus, is not necessary although phosphorus can be added in an amount of up to approximately 0.5 wt.%.
This alloy is preferably formed by a casting method in which the creation of brittle phases is suppressed by rapid cooling from the molten state. These high cooling-off rates are achieved by band casting or spray compacting. Preforms of the alloys of the present invention manufactured by these methods are distinguished by having even, precipitation-poor primary structures. This structural state provides for a high mechanical strength and workability which enables the preforms to be processed without any problems by conventional forming methods. Additionally, the alloy has excellent hard-solderability properties with many different types of solders but does not have the problem of oxides forming on the surface which would cause a poor wetability and solder flow.
In a further embodiment of the present invention, the copper-tin alloy contains tin in an amount of from 12 to 20 wt. % and iron in an amount of from 0.2 to 5 wt. %. In order to achieve a good formability of these alloys, the original forming of the alloy should occur by a casting method in which the creation of brittle phases is avoided by a high cooling rate. It is surprising that during the casting of the alloy of the present invention by such a method, complicated vacuum or protective gas techniques are essentially not required. These alloys are characterized by a high strength or hardness, high resistance to creeping or softening and a high resistance to wear and, on the other hand, still possess a sufficiently high ductility which enables them to be changed in form by cold-forming by a degree of more than 20%. Iron can be partially or completely replaced in this alloy composition with cobalt. Manganese and/or zinc in an amount of up to 5% by weight can also be added to the alloy.
The chipping characteristics of the alloy can be adjusted by the addition of lead or graphite in an amount of up to 3 volume %. The addition of lead or graphite also can provide for improved characteristics in friction or sliding-stressed structural parts. However, the lead or graphite content is limited to 3% by volume in order to avoid negative effects on the forming properties of the alloy. Aluminum in an amount of up to 2.5% by weight can be added to further increase the mechanical strength of the alloy. Higher contents of aluminum are not practical since they adversely influence the surface treatment or subsequent joining of the alloy. Nickel also can be present in the alloy of the present invention in an amount of up to 5% by weight in order to improve the mechanical strength and corrosion resistance of the alloy. However, the addition of nickel in amounts above 5% by weight adversely affect the processability of the alloy by increasing its hardness.
Depending on how the inventive alloy is manufactured, phosphorus can be utilized for the deoxidation of the melt. The phosphorus exhibits a significant effect starting with 0.01 wt. % but to avoid iron-phosphide particles in the alloy structure, the phosphorus content is adjusted to the iron concentration such that the iron content/phosphorus content is greater than 2 and the phosphorus content in the alloy is not above 0.5 wt. % to avoid the reduction of the ductility of the material and the production of the loose adhering layers of scale during the heat processing.